Method for fabricating a semiconductor device

ABSTRACT

A heterojunction structure has an Al x Ga 1−x As layer (0&lt;x≦1), on which an Al y Ga 1−y As layer (0≦y≦1 and y&lt;x) is provided and having a band gap energy smaller than that of the Al x Ga 1−x As layer and a valence band energy edge higher than that of the Al x Ga 1−x As layer. When the Al y Ga 1−y As layer is selectively etched, an Au electrode film is formed on a surface of the Al y Ga 1−y As layer outside an etching region, a resist pattern is formed covering the Au electrode film and leaving exposed the etching region, and the Al y Ga 1−y As layer is selectively removed by etching while irradiating with light, using an etching solution having a Fermi level higher than that of the Al y Ga 1−y As layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for fabricating a semiconductordevice and, more particularly, to an etching method used in the methodfor fabricating a semiconductor device.

2. Description of the Related Art

To constitute various types of semiconductor devices including, forexample, laser diodes, GaAs-FET and HEMT, a hetero junction structurehas been used to improve the characteristics of the semiconductordevice. In order to arrange a good-quality hetero junction structure,the lattice constants of both materials should be substantiallycoincident with each other.

In the field of semiconductor lasers, for example, there has beenproposed the reduction of a threshold current by use of a so-calledheterojunction structure wherein an emission region having a small bandgap energy be sandwiched between semiconductor layers having a largeband gap energy. Since then, it has been known to prepare aheterojunction structure of good quality using GaAs andAl_(x)Ga_(1−x)As, thereby forming a GaAs-AlGaAs double heterojunctionlaser.

The Al_(x)Ga_(1−x)As material exhibits an increasing band gap energy Egwith an; increase in x, and, although the refractive index n decreases,the change in lattice constant is very small.

In the selective etching at the interface of growth of a hetero junctionstructure for the formation of a semiconductor device having the heterojunction structure, the etching is stopped only due to the difference inchemical etching rate between a layer to be etched and a layer forstopping the etching. Accordingly, it is necessary that the ratio inetching rate between the layer to be etched and the layer for stoppingthe etching be 50 or more.

This makes it difficult to stop the etching with high accuracy dependingon the types of materials for the layer to be etched and the layer forstopping the etching in the course of selective etching, even though anAl_(x)Ga_(1−x)As material capable of forming a high-qualityheterojunction is used. This entails much time for the choice of anetching solution and also an appreciable limitation placed on the choiceof materials used to constitute a semiconductor device.

FIG. 13 is a perspective view showing a conventional semiconductorlaser. In FIG. 13, indicated by 100 is a semiconductor laser, by 101 isan n-type GaAs substrate (n-type is hereinafter referred to as “n-”, andlikewise, p-type is referred to as “p-”), by 102 is a buffer layer madeof n-GaAs, by 103 is an n-type clad layer made of n-Al_(0.5)Ga_(0.5)As,by 104 is a multiple quantum well active layer made ofAl_(0.35)Ga_(0.65)As/Al_(0.15)Ga_(0.85)As, by 105 is a first p-type cladlayer made of p-Al_(0.5)Ga_(0.5)As, by 106 is an etching stopper layermade of p-Al_(0.2)Ga_(0.8)As, by 107 is a current block layer made ofAl_(0.6)Ga_(0.4)As, by 108 is an opening of the current block layer 107,by 109 is a surface protective layer made of n-GaAs, by 110 is a secondp-type clad layer made of p-Al_(0.5)Ga_(0.5)As, by 111 is a contactlayer made of p-GaAs, by 112 is a removed region of the contact layer111, by 113 is a p electrode, and by 114 is an n electrode.

Next, a method of fabricating a conventional semiconductor laser isdescribed.

FIGS. 14 and 15 are, respectively, a sectional view showing asemiconductor laser at one stage in a conventional method of fabricatinga semiconductor laser. FIGS. 14 and 15 are, respectively, a sectionalview taken along line XIV—XIV of FIG. 13.

Referring to FIG. 14, after successive deposition, on the n-GaAssubstrate via the buffer layer 102, of the n-type clad layer 103, themultiple quantum well active layer 104, the first p-type clad layer 105,the etching stopper layer 106, the current block layer 107 and thesurface protective layer 109, a resist film is formed on the surface ofthe surface protective layer 109 to form a resist pattern having aband-shaped opening along a direction of an optical waveguide. Thesurface protective layer 109 is subjected to patterning by use of aphotolithographic technique using the resist pattern as a mask.Subsequently, after removal of the mask pattern of the resist film, thecurrent block layer 107 is selectively etched through the mask of thepatterned surface protective layer 109 until the etching stopper layer106 is exposed, thereby forming a band-shaped opening 108 in the currentblock layer 107.

Thereafter, the second p-type clad layer 110 and the contact layer 111are successively built up on the current block layer 107 including theopening 108 and the surface protective layer 109.

A resist film is formed on the surface of the contact layer 111, and aresist pattern 115 having an opening is formed in the vicinity ofopposite end faces of the band-shaped opening 108, followed by selectiveetching of the contact layer 111 through the resist pattern 115 used asa mask to form the removed region 112 of the contact layer 111. Theresults provided by the selective etching step are shown in FIG. 14.

For selective etching and removing the contact layer 111, a mixture ofammonia and hydrogen peroxide is used as an etching solution.

With reference to FIG. 15, the resist patter 115 is removed and the pelectrode 113 is formed, and the n-GaAs substrate 101 is polished at aback side thereof to a given thickness, followed by formation of the nelectrode 114. The results of these steps are shown in FIG. 15.

The etching solution (a mixed solution of ammonia and an aqueoushydrogen peroxide solution) used in the selective etching for removal ofthe contact layer 111 carried out in the conventional fabrication methodserves to stop the etching by using only the difference in chemicaletching rate between GaAs used for the contact layer 111 andAl_(0.5)Ga_(0.5)As used for the second p-type clad layer 110.Accordingly, it is necessary that the ratio of the etching rate betweenthe GaAs of the layer to be etched and the Al_(x)Ga_(1−x)As of theetching stop layer be 50 or over. To this end, it is necessary that thecompositional ratio of Al in the etching stop layer be at 0.2 or over.

In this case, the etching stop layer is constituted of the second p-typeclad layer 110, so that the compositional ratio of Al can be set at 0.5,thereby ensuring a satisfactory etching rate ratio to GaAs. In general,however, such conditions are not always ensured, and thus, the selectiveetching of a compound semiconductor subjected to hetero junction hassuffered a substantial limitation depending on the type ofheterojunctioned material, which has, in turn, placed considerablelimitations on the selection and structure of constituting materials ofa semiconductor device.

Though a satisfactory etching rate ratio has been ensured, the controlof carrying out etching to a necessary and sufficient extent is quitedifficult, under which overetching leads to side etching. If a mixedsolution of ammonia and an aqueous hydrogen peroxide solution isprovided as an etching solution, surface oxidation takes placeviolently, thus being undesirable from the standpoint of surfacemorphology.

It will be noted that known techniques are described in Japanese PatentLaid-Open Nos. Hei 01-099276, Sho 61-077384 and Sho 62-176183, whichdisclose techniques of improving the accuracy of selective etching.

In Japanese Patent Laid-Open No. Hei 01-099276, there is disclosed amethod using tartaric acid as an etching solution.

Moreover, in Appl. Phys. Lett. 55(10), Sep. 4, 1989, p. 984-p. 986,photochemical etching is described wherein a laser beam from aGaAs/AlGaAs hetero structure is irradiated.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-describeddrawbacks and disadvantages of the related art. It is an object of thepresent invention to provide a method for fabricating a semiconductordevice comprising the step of accurately stopping selective etching atthe interface of a hetero junction arrangement.

According to one aspect of the invention, there is provided a method forfabricating a semiconductor device as follows. The method comprises thesteps of; providing a hetero junction structure wherein a firstsemiconductor layer is formed thereon with a second semiconductor layerthat has a band gap smaller than that of the first semiconductor layerand a valence band energy larger than that of the first semiconductorlayer and forming a metal film on a surface of the second semiconductorlayer and outside a first portion where the second semiconductor layeris to be removed; and forming a mask pattern covering the metal film andpermitting the first portion of the second semiconductor layer to beexposed and selectively removing the second semiconductor layer underirradiation of light through the mask pattern as a mask by use of anetching solution having a Fermi level higher than that of the secondsemiconductor layer.

Accordingly, holes contributing to etching are generated by irradiationof light and the second semiconductor layer is rendered thinner. Thusmobility of the holes in directions parallel to the thin film of thesecond semiconductor layer increases and the holes are likely to movetoward the metal film via the second semiconductor, thereby reducing thenumber of holes contributing to the etching and stopping the etching ofthe second semiconductor layer. The etching can be stopped to anecessary and sufficient extent at the interface of the hetero junction,thereby permitting selective etching. Thus, the stop of the etching canbe very accurately controlled. Eventually, this leads to mitigation oflimitations on the type of material and structure of a semiconductordevice and also to inexpensive fabrication of semiconductor deviceshaving uniform characteristics by a simple procedure.

Another object of the invention is to provide a method for fabricatingthe AlGaAs laser having the contact layer-removed structure by a simpleprocedure.

According to another aspect of the invention, there is provided a methodfor fabricating a semiconductor device. The method comprises the stepsof: successively forming, on a GaAs substrate of a first conductiontype, a lower clad layer made of an AlGaAs material of a firstconduction type, an active layer made of an AlGaAs material and having amultiple quantum well structure, a first upper clad layer made of anAlGaAs material of a second conduction type, and a current block layermade of an AlGaAs material of a first conduction type and forming agroove for a current path in the current block layer made of the AlGaAsmaterial along a direction of a light guide; forming a second upper cladlayer made of an Al_(x)Ga_(1−x)As (0<x≦1) on the current block layer soas to bury the groove, and forming thereon a contact layer made of anAl_(y)Ga_(1−y)As (0≦y≦1 and y<x) of a second conduction type; forming ametal electrode film on the contact layer and forming a resist patternso as to cover the metal electrode film therewith and expose a surfaceof the contact layer at opposite ends of the groove for the currentpath; and selectively etching the contact layer with an etchingsolution, which has a Fermi level higher than that of the contact layerand contains tartaric acid, by use of the resist pattern as a mask underirradiation of light.

Accordingly, the selective etching of the contact layer of the AlGaAslaser having a contact layer-removed structure can be performed by asimple process. This eventually leads to the mitigation of limitingconditions of the type of a material and structure constituting theAlGaAs laser having the contact layer-removed structure and also to theinexpensive fabrication of AlGaAs lasers, which have uniformcharacteristics, good surface morphology and the contact layer-removedstructure, by a simple procedure.

Other objects and advantages of the invention will become apparent fromthe detailed description given hereinafter. It should be understood,however, that the detailed description and specific embodiments aregiven by way of illustration only since various changes andmodifications within the scope of the invention will become apparent tothose skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views illustrating a method forfabricating a semiconductor device according to an embodiment of theinvention;

FIGS. 2A and 2B are schematic views illustrating a method forfabricating a semiconductor device according to the embodiment of theinvention;

FIG. 3 is a perspective view of a semiconductor laser according to anexample embodying a method for fabricating a semiconductor device of theinvention;

FIG. 4 is a sectional view of the semiconductor laser of the invention,taken along line IV—IV of FIG. 3;

FIG. 5 is a sectional view of the semiconductor laser of the invention,taken along line V—V of FIG. 3;

FIG. 6 is a sectional view of a semiconductor laser at a step of amethod for fabricating a semiconductor laser according to an embodimentof the invention;

FIG. 7 is a sectional view of the semiconductor laser at another step ofthe method for fabricating a semiconductor laser according to theembodiment of the invention;

FIG. 8 is a sectional view of the semiconductor laser at a further stepof the method for fabricating a semiconductor laser according to theembodiment of the invention;

FIG. 9 is a sectional view of the semiconductor laser at a still furtherstep of the method for fabricating a semiconductor laser according tothe embodiment of the invention;

FIG. 10 is a sectional view of the semiconductor laser at a yet furtherstep of the method for fabricating a semiconductor laser according tothe embodiment of the invention;

FIG. 11 is a sectional view of the semiconductor laser at another stepof the method for fabricating a semiconductor laser according to theembodiment of the invention;

FIG. 12 is a sectional view of the semiconductor laser at yet anotherstep of the method for fabricating a semiconductor laser according tothe embodiment of the invention;

FIG. 13 is a perspective view of a conventional semiconductor laser;

FIG. 14 is a sectional view of a semiconductor laser at one stage in aconventional method for fabricating a semiconductor laser; and

FIG. 15 is a sectional view of a semiconductor laser at another stage inthe conventional method for fabricating a semiconductor laser.

In all figures, the substantially same elements are given the samereference numbers.

DESCRIPTION TO THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B, and FIGS. 2A and 2B are, respectively, schematic viewsillustrating a method of fabricating a semiconductor device according toone embodiment of the invention.

FIG. 1A is a schematic view showing the movement of holes in the courseof etching, and FIG. 1B is a schematic view showing an energy band inthe course of etching.

FIG. 2A is a schematic view showing the movement of holes underconditions where etching is stopped, and FIG. 2B is a schematic viewshowing an energy band under conditions where etching is stopped.

In FIGS. 1A and 2A, reference numeral 10 indicates, for example, anAl_(x)Ga_(1−x)As layer (where 0<x≦1) as a first semiconductor layer, andreference numeral 12 indicates, for example, an Al_(y)Ga_(1−y)As layer(where 0≦y≦1 and y<x).

Reference numeral 14 indicates an etching region where the anAl_(y)Ga_(1−y)As layer 12 is to be etched as a first portion, andreference numeral 16 indicates, for example, an Au electrode film as ametal film. This Au electrode film 16 is formed at a distance of aboutseveral μm to 10 μm from the etching region 14 to surround the etchingregion 14 at an outer side thereof.

Reference numeral 18 indicates a resist pattern provided as a maskpattern, which is so formed as to cover the Au electrode film 16therewith and to expose the etching region 14.

The magnitudes of the energies of the band gap and the valance band ofthe Al_(x)Ga_(1−x)As layer 10 and the Al_(y)Ga_(1−y)As layer 12 are suchthat as will be seen from the Al compositional ratio set as y<x and alsofrom the energy bands shown in FIGS. 1B and 2B, the band gap of theAl_(x)Ga_(1−x)As layer 10 is larger than that of the Al_(y)Ga_(1−y)Aslayer 12, and the energy of the valence band of the Al_(y)Ga_(1−y)Aslayer 12 is larger than that of the Al_(x)Ga_(1−x)As layer 10.

When an etching solution is adjusted so that the Fermi level thereof ishigher than that of the Al_(y)Ga_(1−y)As layer 12, the shapes of theconduction band and the valence band of the Al_(y)Ga_(1−y)As layer 12 incontact with the etching solution can be slightly curved as shown in theenergy bands of in FIG. 1B and FIG. 2B.

The Fermi level of this etching solution is the electrochemicalpotential of the redox electrons in the etching solution, as describedin, for example, a book entitled “Electrode Chemistry” authored by NorioSato and published by Japan Technical Information Service in 1993(Chapter 2, pages 70 through 99).

The electrochemical potential of a redox electron “μ(/)e(REDOX)”, or theFermi level of the redox electron “εF (REDOX)” is generally expressed bythe following formula. It should be noted that the expression “μ (/)”denotes μ with an upper bar.

μ(/)e(REDOX)=εF (REDOX)={(εOX,A+εRED,D)/2}+{(λOX−λRED)/2}

where:

εOX,A is the acceptor level of the oxidant, or the electron affinity ofthe oxidant; εRED,D is the donor level of the reductant, or theionization energy of the reductant; and λ OX and λ RED are the hydrationstructure rearrangement energy of the oxidant and the reductant,respectively.

Selective etching is now described.

FIGS. 1A and 1B show the state where etching is in progress, under whichwhen light having a sufficient energy hν is irradiated, electron andhole pairs are formed at the etching region 14, exposed through theresist pattern 18 used a mask, of the Al_(x)Ga_(1−x)As layer 10 and theAl_(y)Ga_(1−y)As layer 12. The electron and holes formed in theAl_(x)Ga_(1−x)As layer 10 move toward the Al_(y)Ga_(1−y)As layer 12, andespecially, the holes are stored up in the Al_(y)Ga_(1−y)As layer 12. Ata portion which contacts the etching solution through the resist pattern18 as a mask, the holes are combined with the OH group (OH⁻) of theetching solution, thereby permitting the Al_(y)Ga_(1−y)As layer 12 to bedissolved and etched.

FIGS. 2A and 2B show the state where etching is further in progress andthe Al_(y)Ga_(1−y)As layer 12 is made thinner.

As the Al_(y)Ga_(1−y)As layer 12 at the etched region 14 is madethinner, the holes stored up in the Al_(y)Ga_(1−y)As layer 12 areconverted to a two-dimensional hole gas, thereby increasing the mobilityof the thinner Al_(y)Ga_(1−y)As layer 12 in parallel directions.Accordingly, immediately after the holes formed in the Al_(x)Ga_(1−x)Aslayer 10 arrive at the Al_(y)Ga_(1−y)As layer 12, the holes move indirections parallel to the Al_(y)Ga_(1−y)As layer 12, and are movedtoward the Au electrode film 16 via the masked Al_(y)Ga_(1−y)As layer 12and disappear through the Au electrode film 16. As a result, the numberof holes to be combined with the hydroxyl groups of the etching solutionis reduced, so that the dissolution of the Al_(y)Ga_(1−y)As layer 12 isstopped, thereby stopping the etching.

In order to chemically stop the etching in the course of selectiveetching, it has been necessary that the etch rate ratio be at 50 orover. When etching is effected photochemically under satisfactory lightirradiation so that the semiconductor layer to be etched is renderedthin sufficient to permit the holes stored up in the semiconductor layerto be converted to the two-dimensional hole gas, the mobility of theholes along directions parallel to the thin film is increased. In thiscondition, the holes are moved to a metal film formed on the surface ofthe semiconductor layer to be etched, and thus, do not contribute to theetching, under which even though the etch rate ratio is chemicallysmall, the etching can be stopped at an etching stop layer therebyenabling selective etching to be performed. In this way, etching stopcan be very accurately controlled.

This eventually leads not only to mitigation of limiting conditions onthe type of constituting material and structure of a semiconductordevice, but also the inexpensive fabrication of semiconductor devices ofuniform characteristics by a simple procedure.

EXAMPLE 1

A catastrophic optical damage (hereinafter referred to as COD) is knownas a degradation phenomenon of a semiconductor laser. This CODdegradation is ascribed to the rise of a temperature at a laser end faceto an extent of crystal breakage through the interaction of thegeneration of heat at the end face of the laser and the light absorptioncaused by the generation of heat.

To prevent the COD degradation, the suppression of generation of Jouleheat has been proposed by not passing an electric current to the activelayer in the vicinity of the end face of the laser, and a semiconductorlaser that has a contact layer-removed structure wherein part of thecontact layer in the vicinity of the laser end face has been removed isnow put into practice.

FIG. 3 is a perspective view of a semiconductor laser according to anexample of a method of fabricating a semiconductor laser of theinvention. FIG. 4 is a sectional view of a semiconductor laser, takenalong line IV—IV of FIG. 3, and FIG. 5 is a sectional view of asemiconductor laser taken along the line V—V of FIG. 3.

In FIGS. 3, 4 and 5, indicated by 20 is a semiconductor laser having acontact layer-removed structure, by 22 is an n-GaAs substrate, by 24 isa buffer layer provided on the n-GaAs substrate 22 and made of n-GaAs,by 26 is an n-type clad layer provided on the buffer layer 24 and madeof n-Al_(0.5)Ga_(0.5)As, and by 28 is an active layer having a multiplequantum well structure provided on the n-type clad layer 26. A lightguide layer and a barrier layer are, respectively, formed ofAl_(0.35)Ga_(0.65)As, and a well layer is formed ofAl_(0.15)Ga_(0.85)As.

Indicated by 30 is a first p-type clad layer provided on the activelayer 28 and made of p-Al_(0.5)Ga_(0.5)As, and by 32 is an etchingstopper layer provided on the first p-type clad layer and made ofp-Al_(0.2)Ga_(0.8)As.

Indicated by 34 is a current block layer provided on the etching stopperlayer 32 and made of n-Al_(0.6)Ga_(0.4)As, and by 36 is a surfaceprotective layer provided on the current block layer 34 and made ofn-GaAs, with an opening 38 extending along a light guide direction atthe centers of the current block layer 34 and the surface protectivelayer 36. This opening 38 serves as a current path, through which anelectric current passes to the active layer 28.

Indicated by 40 is a second p-type clad layer buried in the opening 38,provided on the surface protective layer 36 and made ofp-Al_(x)Ga_(1−x)As, where x, in this example, equals 0.5. Indicated by42 is a contact layer provided on the second p-type clad layer 40 andmade of p-Al_(y)Ga_(1−y)As (0<y<1 and y<x) where, in this example, y=0.The contact layer 42 is provided with a removal region 44 correspondingto the opening 38 from which the contact layer 42 in the vicinity of thelaser end face has been removed so that no electric current passes tothe vicinity of the laser end face of the active layer 28.

Indicated by 46 is a p electrode constituted of an Au-containing metallayer, which is so arranged that at least a part thereof correspondingto a peripheral edge of the removal region 44 surrounds the outer sideof the removal region 44 at a distance of several μm to about 10 μm fromthe removal region 44. Indicated by 48 is an n electrode provided on theback surface of the n-GaAs substrate 22.

In the semiconductor laser 20 of this arrangement, a bias potential isapplied between the p electrode 46 and the n electrode 48, therebypermitting laser oscillation. Because any current does not pass to theactive layer 28 in the vicinity of the laser end face corresponding tothe removed region 44 of the contact layer 42, the generation of theJoule heat is suppressed in the vicinity of the laser end face of theactive layer 28, so that the COD degradation at the laser end face isprevented.

The method for fabricating the semiconductor laser is now described.

FIGS. 6, 7, 8, 9, 10, 11 and 12 are, respectively, sectional views of asemiconductor laser at individual stages of a method for fabricating asemiconductor laser according to the invention.

Referring now to FIG. 6, there are successively formed, on the n-GaAssubstrate 22 by a crystal growth method such as an MOCVD method, of ann-GaAs layer as the buffer layer 24, an n-Al_(0.5)Ga_(0.5)As layer asthe n-type clad layer 26, an Al_(0.35)Ga_(0.65)As/Al_(0.15)Ga_(0.85)Asmultiple quantum well layer as the active layer 28, ap-Al_(0.5)Ga_(0.5)As layer as the first p-type clad layer 30, ap-Al_(0.2)Ga_(0.8)As layer as the etching stopper layer 32, ann-Al_(0.6)Ga_(0.4)As layer as the current block layer 34, and an n-GaAslayer as the surface protective layer 36. The results are shown in FIG.6.

With reference to FIG. 7, a resist film is subsequently formed on thesurface of the surface protective layer 36 and a resist pattern 50having a band-shaped opening extending along the light guide directionis formed.

The n-GaAs layer used as the surface protective layer 36 is subjected topatterning according to a photolithographic technique using the resistpattern 50 as a mask, thereby forming an opening in the n-GaAs layerserving as the surface protective layer 36. The results are shown inFIG. 7.

Referring to FIG. 8, after removal of the resist pattern 50, then-Al_(0.6)Ga_(0.4)As layer serving as the current block layer 34 isetched through the mask of the n-GaAs layer serving as the surfaceprotective layer 36 formed with the opening therein until thep-Al_(0.2)Ga_(0.8)As layer serving as the etching stopper layer 32 isexposed, thereby forming the opening 38. The results are shown in FIG.8.

With reference to FIG. 9, a p-Al_(0.5)Ga_(0.5)As layer serving as thesecond p-type clad layer 40 and the p-GaAs layer serving as the contactlayer 42 are, successively, formed according to a crystal growth methodon the n-Al_(0.6)Ga_(0.4)As layer serving as the current block layer 34including the opening 38 and also on the n-GaAs layer serving as thesurface protective layer 36. FIG. 9 shows the results.

With reference to FIG. 10, the p electrode 46 is formed on the surfaceof the p-GaAs layer serving as the contact layer 42 such as by vacuumdeposition. The p electrode 46 is formed outside the removed region 44of the p-GaAs layer serving as the contact layer 42. FIG. 10 shows theseresults.

With reference of FIG. 11, a resist film is subsequently formed on thecontact layer 42 with the p electrode 46 being covered therewith,followed by formation of a resist pattern 52 by use of aphotolithographic technique so that the removal region 44 of the p-GaAslayer is exposed. Thereafter, this resist pattern 52 is used as a maskto effect etching under light irradiation by use of a mixed solution oftartaric acid and an aqueous hydrogen peroxide solution (with a ratiobetween tartaric acid and the aqueous hydrogen peroxide solution beingat 4:1). As a consequence, the p-GaAs layer only at the removal region44 is selectively etched.

When etching is effected under irradiation of light having a sufficientenergy hν, electron and hole pairs are formed in the p-GaAs layer of theremoved region 44 exposed through the mask of the resist pattern 52 andalso in the p-Al_(0.5)Ga_(0.5)As layer provided as the second p-typeclad layer 40.

The electrons and holes formed in the p-Al_(0.5)Ga_(0.5)As layerprovided as the second p-type clad layer 40 move toward the removedregion 44 of the p-GaAs layer used as the contact layer 42, andespecially, the holes are stored up in the p-GaAs layer at the removalregion 44 wherein the holes are combined with the hydroxyl groups (OH⁻)of an etching solution at a portion that is in contact with the etchingsolution through the mask of the resist pattern 5. Eventually, thep-GaAs layer at the removal region 44 is dissolved so that the etchingis allowed to proceed.

As the p-GaAs layer at the removal region 44 is rendered thinner as aresult of the etching being in further progress, the holes stored in thep-GaAs layer are converted to a two-dimensional hole gas, therebyincreasing the mobility along directions parallel to the thinned p-GaAslayer at the removal region 44.

Accordingly, upon arrival of the holes, which have been formed withinthe p-Al_(0.5)Ga_(0.5)As layer used as the second p-type clad layer 40,at the p-GaAs layer at the removal region 44, the holes immediately movein directions parallel to the thinned p-GaAs layer at the removal region44 and are moved to the p electrode 46 via the masked p-GaAs layer,followed by extinction via the p electrode 46. As a result, the holesthat are to be combined with the hydroxyl group in the etching solutionare reduced in number, so that the dissolution of the p-GaAs layer atthe removal region 44 is stopped, thereby stopping the etching.

At the time, a very thin p-GaAs layer is left, and this layer can be sothin as to be completely removed in a subsequent pretreatment, notcausing any influence functionally as a semiconductor device.Accordingly, the etching can be stopped substantially to a necessary andsufficient extent.

In this connection, in case where a mixed solution of tartaric acid andan aqueous hydrogen peroxide solution is used in place of the mixedsolution of ammonia and an aqueous in a conventional etching method, theetching rates of GaAs and Al_(0.5)Ga_(0.5)As layer are substantiallyequal to each other, under which the etching is not stopped at theinterface between GaAs and Al_(0.5)Ga_(0.5)As but the Al_(0.5)Ga_(0.5)Aslayer is also etched.

After completion of the etching, the resist pattern 52 is removed. FIG.12 shows the results of this step.

Moreover, after the n-substrate 22 is polished at a back side thereofand shaped to a given thickness, the n electrode 48 is formed on theback side of the n-GaAs substrate 22, thereby completing thesemiconductor laser 20 shown in FIGS. 3, 4 and 5.

This selective etching is not carried out by use of a chemical etchingrate ratio alone, and the stop of the etching can be very accuratelycontrolled. Thus, semiconductor lasers having uniform characteristicscan be fabricated by a simple process and can be provided inexpensivelyalong with an improved yield.

Since the etching solution used is made of the mixed solution oftartaric acid and an aqueous hydrogen peroxide solution, there can befabricated a semiconductor laser having a reduced degree of surfaceoxidation and a good surface morphology.

Although an instance using an AlGaAs material has been described in thisembodiment, similar results are obtained using other type of compoundmaterial including an InGaAs material, an AlGaInP material or anAlGaInAs material as well as other type of semiconductor materialshaving the interface of the hetero junction.

The method for fabricating a semiconductor device according to theinvention contains such steps as illustrated hereinabove, with thefollowing effects.

The method for fabricating a semiconductor device according to theinvention contains the steps of; providing a hetero junction structurewherein a first semiconductor layer is formed thereon with a secondsemiconductor layer that has a band gap smaller than that of the firstsemiconductor layer and a valence band energy larger than that of thefirst semiconductor layer and forming a metal film on a surface of thesecond semiconductor layer and outside a first portion where the secondsemiconductor layer is to be removed; and forming a mask patterncovering the metal film and permitting the first portion of the secondsemiconductor layer to be exposed and selectively removing the secondsemiconductor layer under irradiation of light through the mask patternas a mask by use of an etching solution having a Fermi level higher thanthat of the second semiconductor layer. Accordingly holes contributingto etching are generated by irradiation of light and the secondsemiconductor layer is rendered thinner. Thus mobility of the holes indirections parallel to the thin film of the second semiconductor layerincreases and the holes are likely to move toward the metal film via thesecond semiconductor, thereby reducing the number of holes contributingto the etching and stopping the etching of the second semiconductorlayer. The etching can be stopped to a necessary and sufficient extentat the interface of the hetero junction, thereby permitting selectiveetching. Thus, the stop of the etching can be very accuratelycontrolled. Eventually, this leads to mitigation of limitations on thetype of material and structure of a semiconductor device and also toinexpensive fabrication of semiconductor devices having uniformcharacteristics by a simple procedure.

Further, the first and second semiconductor layers are, respectively,formed of an AlGaAs material, an InGaAs material, an AlGaInP material oran AlGaInAs material. In the selective etching of a semiconductor deviceusing the compound semiconductor layers, the etching of the secondsemiconductor layer can be accurately stopped. Thus, the conditions oflimiting the type of material and the structure of a semiconductordevice using an AlGaAs material, an InGaAs material, an AlGaInP materialor an AlGaInAs material can be mitigated, and semiconductor deiceshaving uniform characteristics can be fabricated inexpensively by asimple procedure.

Moreover, since an etching solution containing hydroxyl groups is usedfor etching, the etching can be effectively carried out throughcombination of holes in the second semiconductor layer with the hydroxylgroups. Eventually, semiconductor devices having uniform characteristicscan be fabricated inexpensively by a simple procedure.

In addition, since the etching solution contains tartaric acid, theoxidation of the etched surface can be reduced in degree. Thus, asemiconductor device having a good surface morphology can be simplyfabricated.

Also, the method for fabricating a semiconductor device according to theinvention contains the steps of: successively forming, on a GaAssubstrate of a first conduction type, a lower clad layer made of anAlGaAs material of a first conduction type, an active layer made of anAlGaAs material and having a multiple quantum well structure, a firstupper clad layer made of an AlGaAs material of a second conduction type,and a current block layer made of an AlGaAs material of a firstconduction type and forming a groove for a current path in the currentblock layer made of the AlGaAs material along a direction of a lightguide; forming a second upper clad layer made of an Al_(x)Ga_(1−x)As(0<x≦1) on the current block layer so as to bury the groove, and formingthereon a contact layer made of an Al_(y)Ga_(1−y)As (0≦y≦1 and y<x) of asecond conduction type; forming a metal electrode film on the contactlayer and forming a resist pattern so as to cover the metal electrodefilm therewith and expose a surface of the contact layer at oppositeends of the groove for the current path; and selectively etching thecontact layer with an etching solution, which has a Fermi level higherthan that of the contact layer and contains tartaric acid, by use of theresist pattern as a mask under irradiation of light. Accordingly, theselective etching of the contact layer of the AlGaAs laser having acontact layer-removed structure can be performed by a simple process.This eventually leads to the mitigation of limiting conditions of thetype of a material and structure constituting the AlGaAs laser havingthe contact layer-removed structure and also to the inexpensivefabrication of AlGaAs lasers, which have uniform characteristics, goodsurface morphology and the contact layer-removed structure, by a simpleprocedure.

While the presently preferred embodiments of the present invention havebeen shown and described. It is to be understood these disclosures arefor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

The entire disclosure of a Japanese Patent Application No. 2001-155138,filed on May 24, 2001 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

What is claimed is:
 1. A method for fabricating a semiconductor devicecomprising: on a heterojunction structure including a firstsemiconductor layer, forming a second semiconductor layer that has aband gap energy smaller than the first semiconductor layer, and a highervalence band energy than that of the first semiconductor layer; forminga metal film on a surface of the second semiconductor layer and outsidea first portion of the second semiconductor layer where the secondsemiconductor layer is to be removed; forming a mask pattern coveringthe metal film and exposing the first portion of the secondsemiconductor layer exposed, and selectively removing the secondsemiconductor layer by etching, while irradiating the secondsemiconductor layer with light, with a solution having a Fermi levelhigher than that of the second semiconductor layer, selective removal ofthe second semiconductor layer substantially stopping at the firstsemiconductor layer while irradiating the first and second semiconductorlayers with light.
 2. The method for fabricating a semiconductor deviceaccording to claim 1, wherein the first and second semiconductor layersare selected from the group consisting of AlGaAs, InGaAs, AlGaInP,AlGaInAs.
 3. The method for fabricating a semiconductor device accordingto claim 2, wherein the etching solution contains hydroxyl groups. 4.The method for fabricating a semiconductor device according to claim 3,wherein the etching solution contains tartaric acid.
 5. The method forfabricating a semiconductor device according to claim 1, wherein theetching solution contains hydroxyl groups.
 6. The method for fabricatinga semiconductor device according to claim 5, wherein the etchingsolution contains tartaric acid.
 7. A method for fabricating asemiconductor device, the method comprising: successively forming, on aGaAs substrate having a first conductivity type, a lower cladding layerof AlGaAs having the first conductivity type, an active layer of AlGaAsand having a multiple quantum well structure, a first upper claddinglayer of AlGaAs having a second conductivity type, and a currentblocking layer of AlGaAs having the first conductivity type, and forminga groove for a current path in the current blocking layer, along adirection of a light guide; forming a second upper cladding layer ofAl_(x)Ga_(1−x)As (0<x≦1) on the current blocking layer, filling thegroove, and forming thereon a contact layer made of Al_(y)Ga_(1−y)As(0≦y≦1 and y<x) having the second conductivity type; forming a metalelectrode film on the contact layer; forming a resist pattern coveringthe metal electrode film and leaving exposed a surface of the contactlayer at opposite ends of the groove for the current path; andselectively etching the contact layer with an etching solution which hasa Fermi level higher than that of the contact layer and containstartaric acid, using the resist pattern as a mask, while irradiating thecontact layer with light.